If you hold the mouse down while you power the Mac on (as in, hold it in while you press the power button) it will eject the CD drive. On some Macs the F12, Eject or Shift key can be held down instead.
Then, to boot from a CD, hold the "C" button down while you power the Mac on.
There's a tiny hole on most CD players where you can insert a bent paperclip to force the tray out. If it's a slot-loading drive and there's a disc inserted, you can hold down the mouse button when the Mac starts and the disc will eject. Not sure if the mouse button trick works on tray-loading drives with no disc inserted, but you could try if you don't find a hole for a paperclip. What kind of Mac is it?